Variable fiber optic attenuators play an important role in the implementation of modern information networks having optical interconnects. For instance, in wave division multiplexed (WDM) optical networks having increased wavelength content and greater functionalities, optical channels of communication may be added, dropped, and/or rerouted at any node of the network. This flexibility makes the network more complex from an optical content point of view, such that it becomes important to carefully monitor the optical power in individual wavelength channels. Without such control, unacceptably high error rates may occur during propagation through communication channels having optical amplifiers, so called add-drop modules, multiplexers/de-multiplexers as well as other optical signal conditioning components. Accordingly, low cost, reliable devices to adjust the power level of the optical signals with high accuracy and high repeatability are needed.
Conventional variable optical attenuators have featured several types of architectures, including the use of a lossy fusion splice (see U.S. Pat. No. 4,557,557) and those that vary the distance from one optical fiber end-face to the end-face of an opposed optical fiber (see U.S. Pat. No. 5,050,956). More recently, an attenuator has been developed by Molecular Optoelectronics Corp. (MOEC) that allows electronic control of the attenuation by controlling the temperature of a control layer with an index of refraction that varies with temperature. The control layer is placed against the polished side (cladding) of a fiber to alter the boundary conditions of the optical signals traveling through the core of the fiber , resulting in the attenuation of the optical signal. The fiber side polishing technique, however, results in a rather expensive product due to the manufacturing process which is time and labor intensive.